I am going to show you a few things that might make tracing your car's history a little easier.
Understanding the 1968 new car warranty might be a good place to start.
The 1968 Shelby American Automobile Club Registrar has warranty files on 1968 Shelbys.

The warranty for your 1968 Shelby really starts here. This is the back of the driver's door.
The warranty tag provided some basic information to the dealership and Ford, when your car was serviced.

At the beginning of production, some cars had a larger, oversized warranty tag.
Most cars have the bottom half of the tag snapped off. It was attached to
a "temporary" owner's card.

When new, the temporary owner's card was in the glovebox along with the owner's manual.

Some dealers stored everything in an envelope like this example.

Your owner's manual was designed to answer common questions.
It has a copy of the warranty in the back. For most 1968 Shelbys, that was for
a period of 1 year.

The temporary card was only used until a more permanent plastic card was sent to the owner.

Here are the 1969 warranty cards. The cards were different each year.

The dealer had this bulletin to decode the warranty card.

They also had a quick reference guide.

Service bulletins from Shelby assisted the dealers with common repairs. There were 14 service bulletins.